A typical LCD has the advantages of portability, low power consumption, and low radiation. The LCD has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. However, liquid crystal in the liquid crystal display does not itself emit light. Rather, the liquid crystal has to be lit up by a light source such as a cold cathode fluorescent lamp (CCFL) so as to clearly and sharply display text and images. Generally, the CCFL needs an inverter circuit to supply working voltages.
Referring to FIG. 3, a typical inverter circuit 1 includes a full-bridge circuit 11, a plurality of inverse transformers 12, a plurality of over-voltage protective circuits 13, a plurality of CCFLs 14, and a feedback circuit 15.
The full-bridge circuit 11 includes a first output 111, a second input 112, an over-voltage protective terminal 113, and a feedback terminal 113.
Each inverse transformer 12 includes a first input 121, a second input 122, and a high voltage output 123. The first input 121 is connected to the first output 111 of the full-bridge circuit 11. The second input 122 is connected to the second output 112 of the full-bridge circuit 11. The high voltage output 123 is connected to the over-voltage protective terminal 113 of the full-bridge circuit 11 via a corresponding over-voltage protective circuit 13.
Each CCFL 14 includes a first electrode 141 and a second electrode 142. The first electrode 141 is connected to the high voltage output 123 of a corresponding inverse transformer 12. The second electrode 142 is connected to the feedback terminal 114 of the full-bridge circuit 11.
The feedback circuit 15 includes a plurality of sampling units 151 and an integral circuit unit 156. The number of the sampling units 151 is equal to the number of the CCFLs 14, and each sampling unit 151 corresponds to a respective CCFL 14. Each sampling unit 151 includes a first diode 152, a second diode 153, a third diode 154, and a first resistor 155. The cathode of the first diode 152 is connected to the second electrode 142 of the corresponding CCFL 14, and the anode of the first diode 152 is connected to ground. The anode of the second diode 153 is connected to the second electrode 142 of the corresponding CCFL 14, and the cathode of the second diode 153 is connected to ground via the first resistor 155. The anode of the third diode 154 is connected to the cathode of the second diode 153, and the cathode of the third diode 154 is connected to the integral circuit unit 156. Because of a diode's specific characteristic of one-way electrical conduction, the third diode 154 can prevent sampling voltages of other sampling units 151 from being applied to the cathode of the second diode 153 of the sampling unit 151 and affecting sampling of the sampling unit 151. The integral circuit unit 156 includes a second resistor 157, a third resistor 158, and a capacitor 159. The third resistor 158 and the capacitor 159 are connected in series, and the combination of the third resistor 158 and the capacitor 159 is connected with the second resistor 157 in parallel, thereby forming an integral circuit. The integral circuit can integrate voltages outputted by the sampling units 151, and provide integrated voltages to the feedback terminal 114 of the full-bridge circuit 11.
The full-bridge circuit 11 is configured to convert an external direct current (DC) voltage into an alternating current (AC) voltage, and output the alternating current voltage to the first and second inputs 121, 122 of the inverse transformers 12 through the first output 111 and the second output 112 respectively. The full-bridge circuit 11 pre-sets a safety voltage value and a stable voltage value. The inverse transformers 12 are configured to transform an AC low voltage into an AC high voltage, and output the AC high voltage through the high voltage outputs 123 thereof. The AC high voltage is resonated into a sinusoidal AC voltage under the action of a leakage inductance effect of the inverse transformers 12 and an equivalent capacitance effect of the over-voltage protective circuits 13 and the CCFLs 14. The sampling units 151 of the feedback circuit 15 sample the sinusoidal AC voltage at the second electrodes 142 of the CCFLs 14. The positive half periods of the sinusoidal AC voltage are taken as a sampling voltage, and the negative half periods of the sinusoidal AC voltage are connected to ground. The sampling voltage is integrated by the integral unit 156, and then is sent to the feedback terminal 114 of the full-bridge circuit 11. If the feedback voltage is greater than the stable voltage value, the full-bridge circuit 11 decreases the AC voltage outputted therefrom. If the feedback voltage is less than the stable voltage value, the full-bridge circuit 11 increases the AC voltage outputted therefrom. Thus, a voltage applied to the CCFLs 14 is stabilized.
At the same time, the sinusoidal AC high voltage is inputted to the over-voltage protective circuits 13. If the sinusoidal AC high voltage is less than the safety voltage value, a feedback voltage of the over-voltage protective circuits 13 does not affect working of the full-bridge circuit 11. If the sinusoidal AC high voltage is greater than the safety voltage value, the feedback voltage of the over-voltage protective circuits 13 switches off the full-bridge circuit 11 and thus switches off the inverter circuit 1, in order to protect the CCFLs 14.
In the inverter circuit 1, the feedback circuit 15 feeds back every voltage at the second electrodes 142 of the CCFLs 14, and the number of sampling units 151 is equal to the number of CCFLs 14. If the number of CCFLs 14 is great, the number of the sampling units 151 is correspondingly great. In such case, the structure of the inverter circuit 1 is complicated, and the cost of the inverter circuit 1 is correspondingly high.
What is needed, therefore, is an inverter circuit that can overcome the above-described deficiencies. What is also needed is a liquid crystal display employing such inverter circuit.